The present invention relates to an improved diesel fuel combustion system which produces an exhaust gas containing reduced amounts of smoke and nitrogen oxide.
U.S. Pat. No. 2,331,386 describes addition of an alkyl carbonate such as dibutyl carbonate to liquid fuels of the type employed in oil furnaces and those used in internal combustion automotive engines, particularly to high flash point spark-ignition engine, to impart improved fluidity and anti-knock value to the fuels. It is further described that the dialkyl carbonate blended with a Diesel fuel shows no appreciable alteration in the cetane number of the fuel.
U.S. Pat. Nos. 4,891,049 and 4,904,279 describe that hydrocarbon fuels heavier than gasoline, especially diesel fuel compositions, containing carbonate additives, preferably non-aromatic, metals-free carbonates, to reduce particulate emissions therefrom when combusted in an internal combustion engine.
PCT International Publication WO 84-02339 describes that an internal combustion engine fuels, preferably gasoline, containing a combination of t-butanol and a dialkyl carbonate is effective to significant increase in both Research Octane Number (RON) and Motor Octane Number (MON).
It has been now discovered by the present inventors that production of both smoke and nitrogen oxide in an exhaust gas of a diesel engine can be significantly reduced by employing a combination of a diesel fuel composition containing a carbonic acid ester, particularly, a dialkyl carbonate, and a specifically selected diesel engine operating condition.
Accordingly, it is an object of the invention to provide an improved diesel fuel combustion system which produces an exhaust gas containing a reduced amount of smoke as well as a reduced amount of nitrogen oxide.
The present invention resides in a diesel fuel combustion system which comprises burning a diesel fuel composition comprising a major portion of diesel fuel and 0.1 to 40 vol. % of a carbonic acid diester of the formula: 
in which R represents a lower hydrocarbon group having 1 to 6 carbon atoms
in a diesel engine by injecting the diesel fuel composition into its combustion room at an injection timing of 17 to 2xc2x0 CA BTDC (i.e., Crank Angle Before Top Dead Center).
The above diesel fuel combustion system is particularly favorably employable in EGR system (i.e., exhaust gas recirculation system).
Accordingly, the present invention further provides a diesel fuel combustion system which comprises burning a diesel fuel composition comprising a major portion of diesel fuel and 0.1 to 40 vol. % of a carbonic acid diester of the formula: 
in which R represents a lower hydrocarbon group having 1 to 6 carbon atoms
in a diesel engine by injecting the diesel fuel composition into its combustion room at an injection timing of 17 to 2xc2x0 CA BTDC; and
returning a portion of its exhaust gas produced by the above burning into the combustion room to burn the returned exhaust gas together with a freshly injected diesel fuel composition.